Metal halide lamps are known in the art and are described, for example, in EP0215524, WO2006/046175 and WO05088675. Such lamps operate under high pressure and comprise ionizable gas fillings of, for example, NaI (sodium iodide), TlI (thallium iodide), CaI2 (calcium iodide), and/or REIn. REIn refers to rare earth iodides. Such lamps, when having a ceramic discharge vessel, are also indicated as ceramic discharge metal (CDM) halide lamps.
Characteristic rare earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3, and LuI3. An important class of metal halide lamps are ceramic discharge metal halide lamps (CDM-lamps), which are described in the above-mentioned documents.
WO05088675 for instance discloses a metal halide lamp comprising a discharge vessel surrounded with clearance by an outer envelope and having a ceramic wall which encloses a discharge space filled with a filling comprising an inert gas, such as xenon (Xe) and an ionizable salt, said discharge space accommodating two electrodes arranged such that their tips have a mutual interspacing so as to define a discharge path between them, and a special feature of the ionizable salt being that said ionizable salt comprises NaI, TlI, CaI2 and X-iodide, wherein X is selected from the group comprising rare earth metals. In a specific embodiment of WO05088675, X is one or more elements selected from the group comprising Ce, Pr, Nd.
High intensity discharge lamps may also be metal vapour-based, such as sodium-based (also indicated as high pressure sodium (HPS)). Such lamps are for instance described in GB1582115, GB1587987 and GB2083281. GB1587987 for instance describes a high-pressure sodium vapour discharge lamp provided with a discharge tube which solely contains sodium, mercury and xenon, the sodium vapour pressure in the operating condition of the lamp being between 100-200 Torr, and the xenon pressure at 300 K being between 50 and 1000 Torr (1 Torr=133 Pa or 0.00133 bar).
The use of auxiliary means for initiating the discharge within the discharge vessel of discharge lamps is for instance described in U.S. Pat. No. 5,541,480. This document describes a high-pressure discharge lamp provided with a discharge vessel with a ceramic wall which has an outer surface on which a metallic coating is present. The coating is a metal layer sintered on the ceramic wall, which sintering process takes place during sintering of the discharge vessel so as to achieve translucence. The metal layer is a strip extending along the length dimension of said discharge vessel to facilitate ignition of a discharge within said discharge vessel. The discharge vessel includes a pair of opposing discharge electrodes, each situated at an opposing respective end thereof, and the metal layer may further include a substantially closed circumferential ring extending at the axial location of each electrode and in contact with said strip.